1. Field of the Invention
The present invention relates to a resin double-helical gear.
2. Description of the Related Art
Conventionally, a resin helical gear is widely used as a power transmission mechanism in automobile components, precision machinery, various electronic devices, and the like, in addition to office equipment, such as copiers, printers, and facsimile machines. This is because the resin helical gear has the following advantages attributed a structural characteristic of the resin helical gear in that teeth of the resin helical gear are twisted. The advantages are that power transmission can be performed quietly because meshing is smooth even during high-speed rotation, and a high-load power transmission can be performed because load can be easily dispersed along a tooth trace.
At the same time, the resin helical gear has a following disadvantage because the teeth are twisted. In other words, an axial-direction thrust load proportional to a transmission torque is generated during power transmission. Therefore, a structure for receiving the thrust load is required. To resolve this disadvantage, a configuration in which the thrust load is received by a thrust bearing can be considered. However, in actuality, it may not be possible to dispose the thrust bearing because of spatial constraints and structural constraints of the power transmission mechanism.
Therefore, a technology has been developed for forming a following resin double-helical gear by injection-molding. Like the resin helical gear, the resin double-helical gear is quiet and capable of performing high-speed and high-load power transmission. Moreover, unlike the resin helical gear, the thrust load is not generated in the resin double-helical gear during power transmission (refer to Patent Literature 1).
A resin double-helical gear 30 in Patent Literature 1 is formed by injection-molding, through use of two separate molds, as shown in FIG. 9. In other words, a first mold 33 and a second mold 35 are butted against each other. The first mold 33 forms a section from a tooth width center section 31 to one tooth width direction end side (first gear section 32). The second mold 35 forms a section from the tooth width direction center section 31 to another tooth width direction end side (second gear section 34). A butted section between the first mold 33 and the second mold 35 forms a cavity 36. Molten resin is injected into the cavity 36, and cooled and hardened within the cavity 36. The first mold 33 and the second mold 35 are then separated. The hardened resin is removed from the cavity 36. A large number of resin double-helical gears 30 can be formed within a short amount of time by injection-molding such as this.    Patent Literature 1: Japanese Patent Laid-open Publication No. Heisei 10-315344
However, when the resin double-helical gear is formed as described above, by two separate molds (the first mold 33 and the second mold 35) being butted against each other and molten resin being injected into the cavity 36 formed by the molds 33 and 35, a misalignment may occur on a butted surface 37 between the molds 33 and 35. When even the slightest misalignment is present, a misalignment L2 occurs on a connection surface 3 (a surface corresponding to the butted surface 37) between the first gear section 32 and the second gear section 34 of the injection-molded resin double-helical gear 30. An uneven section (misalignment) 17 is formed in a peak section 7g of an angular tooth 38 and a valley section 7h on the back surface side of the peak section 7g. When the uneven section 17 is formed in the peak section 7g and the valley section 7h of the angular tooth 38, power transmission is performed by only half of the tooth in the tooth width direction (the first gear section 32 or the second gear section 34). The same thrust load as that in the helical gear is generated.
When the uneven section 17 is formed and the resin double-helical gear 30 meshes with a mating resin double-helical gear, contact between tooth surfaces is not performed smoothly at the uneven section 17, thereby causing noise.
Resin double-helical gears are recently being widely used as the power transmission mechanism in the above-described office equipment, automobile components, precision machinery, various electronic devices, and the like. Rotation is not limited to one direction. The resin double-helical gear is often used in situations requiring the resin double-helical gear to rotate in the other direction and in situations in which a driving gear and a driven gear rotate in opposite directions. Therefore, there is a strong demand for suppression of the thrust load and noise caused by the uneven section 17, such as that described above, in both rotation directions (one direction and the other direction) of the resin double-helical gear.